In recent years, studies have been extensively conducted to develop a white light-emitting device using a blue light-emitting element as a light-emitting source. In particular, a white light-emitting device using a blue light-emitting element not only provides a long service life and consuming less electric power compared to an incandescent lamp or a fluorescent lamp, but also uses no hazardous substance such as mercury, and hence a lighting apparatus using a white light-emitting device is becoming practical use at present.
A method most frequently used to obtain white light using a blue color of a blue light-emitting element as a light source is a method of obtaining a pseudo-white color by mixing blue with yellow, which is the complementary color of blue.
For example, in a typical white light-emitting device, a blue light-emitting element is sealed with a transparent resin containing a yellow light-emitting phosphor (for example, Ce-containing YAG (Y3Al5O12) phosphor). A blue light (a wavelength: 450 to 460 nm) is emitted from this blue light-emitting element, and YAG is excited by part of the blue light, and as a result a yellow light is emitted from this YAG phosphor.
When a blue light-emitting element is sealed with a light transmitting resin material such as an epoxy resin, however, it is known that deterioration such as yellowing occurs, which is caused by light emitted from the light-emitting element on light from the outside. In addition, if high current is used in order to obtain high brightness in a white light-emitting device, deterioration of sealing resin occurs owing to heat generated by an element itself. Moreover, the efficiency of light-emitting is reduced owing to the moisture absorption of the sealing resin and the like in some cases.
Considering such circumstances, the present inventors have proposed a white light-emitting device composed of a blue light-emitting element and a ceramic composite for light conversion consisting of a solidified body in which a plurality of oxide crystal phases including an Al2O3 phase and a Ce-containing YAG fluorescent substance phase that emits fluorescence are continuously and three-dimensionally entangled with each other (Patent Literatures 1 and 2). A ceramic composite for light conversion enables stably emitting homogeneous yellow fluorescence owing to uniform distribution of a fluorescent substance phase, and is excellent in durability since it is ceramic. Thus, the problem which occurs when the composite is sealed with an epoxy resin and the like is solved and it is possible to provide a white light-emitting device with high reliability.
The white light-emitting device using this ceramic composite for light conversion is composed of, for example, a blue light-emitting element to be flip-chip mounded, a circuit board having a wiring pattern formed thereon, which performs power receiving and supplying with respect to the blue light-emitting element, and a ceramic composite for light conversion directly bonded with the blue light-emitting element.
In addition, hitherto, a substrate for a light-emitting diode obtained by laminating a single crystal layer capable of fabricating a light-emitting diode element and a ceramic composite layer for light conversion consisting of a solidified body in which at least two or more of oxide crystal phases selected from a monometal oxide and a composite metal oxide are continuously and three-dimensionally entangled with each other, is proposed (Patent Literature 3).
As such a substrate for fabricating a light-emitting diode element, a single crystal substrate such as sapphire is generally used (Patent Literature 4). The method for finishing the surface of this single crystal substrate as an even surface without polishing marks such as scratches generated during polishing includes, in general, a mechanical polishing (hereinafter, referred to as “MP”), such as grinding and lapping, in which the particle diameter of an abrasive grain such as diamond, which is a harder material than a material to be processed, is decreased stepwise, and a chemical mechanical polishing (hereinafter, referred to as “CMP”) which is performed using an abrasive grain softer than a material to be processed, such as colloidal silica, and a polishing liquid involving a chemical action. Thus it is known that, by polishing according to MP, CMP, and the like, the surface of the single crystal substrate becomes a smooth surface with an arithmetic mean roughness (Ra) of less than 1 nm.